The internal combustion engine is operated with direct fuel injection, provision being made for a retardation of the ignition angle as well as an apportionment of the fuel injection into at least two partial injections of fuel, which occur before ignition. A torque loss which results from this measure is compensated by means of an increased charge of the combustion chambers. In the case of a cold catalytic converter, which cannot yet convert, provision if need be is made to establish the air ratio lambda >1, i.e. to operate the internal combustion engine with a lean fuel/air mixture, in order to achieve small emissions of untreated exhaust gas by the internal combustion engine.
Such a method and such a control unit are already used in series production. When the catalytic converter is cold, for example after cold starting the internal combustion engine, the strategy pursued with the known method is to produce a heat flow volume in the exhaust gas, which is as large as possible, without changing the power output of the internal combustion engine or the idle speed, which is elevated if need be in the post-starting phase, at approximately 1,200/min.
This is achieved in the most frequently applied method as a result of a first part of the fuel quantity being injected during the intake stroke and a second part of the fuel quantity during the compression stroke. As a result, a stratified fuel distribution occurs in the combustion chamber with a zone, which arises from the injection of the second part, with a comparatively rich and therefore very ignitable fuel/air mixture around the spark plug.
This operation mode of the internal combustion engine can be denoted as a homogeneous split operation mode, “split” referring to the apportionment of the fuel injection.
The charge stratification allows for a very late ignition timing in the range of 10 to 30° of crankshaft rotation after TDC (TDC=top dead center) when the engine rotational speed is stable and the untreated exhaust gas emissions are controllable. The retarded ignition timing leads to a relatively poor ignition angle efficiency, the ratio of the torques produced at the retarded ignition point and at an optimum ignition point being understood here. The torque loss which results from the poor ignition angle efficiency is compensated by means of an increase in the charges of the combustion chambers of the internal combustion engine. Increases in the charges of the combustion chambers up to values, which amount to approximately 75% of the possible maximum charge under normal conditions, occur at the ignition angles, which are implemented. In total a relatively large quantity of exhaust gas arises thereby, whose temperature is relatively high due to the poor ignition angle efficiency, so that a maximum heat flow (enthalpy flow) occurs in the exhaust-gas region.
When heating with a maximization of the exhaust-gas enthalpy, the exhaust-gas region has to be completely heated from the exhaust valve forward up until the catalytic converter. The heating capacity of these components leads, in particular in internal combustion engines with exhaust-gas turbochargers, to large heat losses before the catalytic converter, which impede an effective heating of the catalytic converter. It is additionally problematic with internal combustion engines with exhaust-gas turbochargers, in that during a maximization of the exhaust-gas enthalpy, the exhaust manifold lying in the jetway of the exhaust gases before the turbine of the turbocharger is heated very quickly to temperatures, whereat an additional heating can lead to the destruction of said manifold. This limits the maximization of the exhaust gas enthalpy, which is desired for heating the catalytic converter.
The homogeneous split operation mode of the internal combustion engine previously described can be employed in a post-starting phase with a constant timing of the point of injection times and the ignition points. In conventional engine management systems of internal combustion engines, the post-starting phase then begins after an actuation of the starter if the rotational speed of the internal combustion engine exceeds a rotational speed threshold value, which lies between the starter rotational speed and the rotational speed of the engine at idle, and then lasts over a predetermined time period of normally 20 to 30 seconds. Within this time period, a precatalytic converter disposed close to the catalytic converter normally achieves an operating temperature (light-off temperature), whereat the pollutant conversion, in particular the conversion of hydrocarbons, noticeably starts. According to a usual definition, the light-off temperature corresponds to that temperature, whereat 50% of the undesirable exhaust gas components, which emerge before the catalytic converter and include carbon monoxide (CO) and hydrocarbons (HC), are converted into non-toxic elements like water and carbon dioxide.
In reality, the percentage of pollutant conversion does not rise dramatically, but rises rather gradually. After the start of the pollutant conversion in the precatalytic converter, the measurable concentration of hydrocarbons downstream of the precatalytic converter quickly drops to values near zero. As has been shown in tests, the drop in the hydrocarbon concentration downstream of the precatalytic converter correlates to the light-off temperature being achieved in a central region of the precatalytic converter. The quantity of hydrocarbons emitted into the environment after cold starting the internal combustion engine or, for example, after an overrun fuel cut-off (operation of the internal combustion engine without fuel metering) is for that reason very much dependant on the time span, which is necessary for achieving the operating temperature of the catalytic converter.